Hitherto, a technique of recording color images in facsimile printers, copying machines, televisions, etc., has been desired and color recording techniques by electrophotography, ink jet printing, thermal transfer recording, etc., have been investigated.
A thermal transfer system is considered to be advantageous as compared with other systems since in the thermal transfer recording system, the management and operation of the apparatus can be easily performed and also the apparatus and supplies for the apparatus are inexpensive.
For the thermal transfer system, there are a melt system of heating a transfer sheet having a heat-melting ink layer formed on a base film by a thermal head to melt the ink and transfer-recording the ink on a recording material and a sublimation system of heating a transfer sheet having an ink layer containing sublimable dyes formed on a base film to sublime the dye and transfer-recording onto a recording material.
Since in the sublimation system, the sublimed and transferred amount of dyes can be controlled by changing the energy applied to a thermal head, gradation recording can be easily performed and hence the system is considered to be particularly advantageous for full color recording.
In thermal transfer recording of the sublimation system, the sublimable dye(s) which is used for the transfer sheet are very important since they give large influences on the speed of transfer recording, the image quality of recorded images, the storage stability of images formed and are required to satisfy the following factors:
(1) The dye easily sublime by the working condition of an ordinary heating means such as thermal recording head.
(2) The dye does not cause thermal decomposition under the working condition of a thermal recording head before subli- mation.
(3) The dye has a preferred hue for color reproduction.
(4) The dye has a large molecular extinction coefficient.
(5) The dyes are stable to heat, light, moisture, chemicals, etc.
(6) The dye can be easily synthesized.
(7) The dye has an excellent ink-forming aptitude.
(8) The dye gives no problems on safety and hygiene.